The present invention relates to a method for making a disposable diaper with each of laterally opposite sides of a waist being welded.
One example of a pant type of disposable diapers is disclosed in Japanese Patent Application Disclosure No. 199315551, according to which a diaper comprising a topsheet, a backsheet, and a liquid-absorbent core sandwiched therebetween is folded up along a transverse center line passing through a crotch zone to lay front and rear bodies one upon another, and the top- and backsheets are ultrasonically welded together along laterally opposite side edges at its waist level to obtain a pant type configuration. The top- and backsheets may be prepared, for example, from nonwoven fabric of thermoplastic synthetic resin, both of which sheets can be molten together by ultrasonic treatment.
If the same kinds of nonwoven fabrics are used for the top- and backsheets in said Disclosure and molten at the same temperature to exhibit viscosities appropriate for welding, the welded top- and backsheets at the temperature may present a smoothly finished appearance. However, if the topsheet prepared from nonwoven fabric of polypropylene having a relatively high melting point is attached to the backsheet prepared from a polyethylene sheet having a relatively low melting point, the backsheet is molten earlier than the topsheet when the basic diaper is folded up like the joined hands and subjected to an ultrasonic welder. In consequence, the polyethylene of the backsheet may be molten to an excessively low viscosity and stick to a horn which has been pressed against the backsheet. The polyethylene sheet which has once sticked to the horn has, after cooled, a roughened surface giving a wearer of the diaper skin itchy stimulus as well as uncomfortable feel and a bad aesthetic appearance of the diaper. If a bit of the polyethylene sheet remains sticking to the horn, no accurate clearance can be assured between the horn and the anvil of the welder during the next welding cycle, making a continuous welding difficult.
Accordingly, it is a principal object of the invention to solve problems accompanying said prior art by preparing a backsheet in diaper's wing-like portions from a sheet having a melting point higher than a melting point of a topsheet thereof.